Before and after the explosion. Colour coded elements in artist's impression of the star's core match up with real observations of Cassiopeia A. Iron, shown in blue, has somehow gone from being in the middle of the star to the outskirts of the supernova remnant.

IN THE NORTHERN CONSTELLATION Cassiopeia lies a famous supernova remnant, a gas cloud that represents the shattered remains of a once-titanic star.

Known as Cassiopeia A, it is hard to see at normal visible light wavelengths but shines strongly at radio and X-ray wavelengths.

A supernova of this kind occurs when a massive star runs out of nuclear fuel and can no longer produce energy. Without an outflow of energy to keep the star “inflated”, it’s huge mass crushes inwards … producing an implosion followed by an explosion that tears the star apart.

In the millions of years leading up to the explosion, the star’s innards become layered with different elements as a result of different nuclear fusion processes. In the core there is a globe of iron. Surrounding that are layers of sulphur, silicon, magnesium, neon and oxygen.

It would seem to make sense that when the star explodes, the outer layers would end up on the outskirts of the resulting gas cloud, with the iron concentrated toward the middle.

But by compiling a staggering one million seconds worth of X-ray observations by satellite telescopes, astronomers have found that this is not the case for Cassiopeia A.

The observations show that the distribution of sulphur, silicon, magnesium and neon is about as expected. But the iron is spread around the outer part of the supernova remnant, and there’s none in the middle.

The astronomers think there must have been some sort of “instability” in the supernova explosion process, which turned the star inside out.

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